High-Q Si3N4 microresonators based on a subtractive processing for Kerr nonlinear optics

TL;DR

This paper reports the fabrication of ultra-smooth silicon nitride microresonators with extremely high optical quality factors, enabling the generation of soliton microcombs with photodetectable repetition rates for integrated photonic applications.

Contribution

The authors demonstrate a subtractive processing method to produce Si3N4 microresonators with Qs around 11 million, surpassing previous limitations and enabling new integrated photonic functionalities.

Findings

Achieved mean intrinsic Qs of ~11 million in Si3N4 microresonators.

Engineered cross-section geometry for normal and anomalous dispersion.

First demonstration of photodetectable repetition rate soliton microcombs in Si3N4.

Abstract

Microresonator frequency combs (microcombs) are enabling new applications in frequency synthesis and metrology from high-speed laser ranging to coherent optical communications. One critical parameter that dictates the performance of the microcomb is the optical quality factor (Q) of the microresonator. Microresonators fabricated in planar structures such as silicon nitride (Si3N4) allow for dispersion engineering and the possibility to monolithically integrate the microcomb with other photonic devices. However, the relatively large refractive index contrast and the tight optical confinement required for dispersion engineering make it challenging to attain Si3N4 microresonators with Qs > 10 000 000 using standard subtractive processing methods. In this work, we achieve ultra-smooth Si3N4 microresonators featuring mean intrinsic Qs around 11 million. The cross-section geometry can be precisely engineered in the telecommunications band to achieve either normal or anomalous dispersion, and we demonstrate the generation of mode-locked dark-pulse Kerr combs as well as soliton microcombs. Such high-Qs allow us to generate soliton microcombs with photodetectable repetition rates, demonstrated here for the first time in Si3N4 microresonators fabricated using a subtractive processing method. These results enhance the possibilities for co-integration of microcombs with high-performance photonic devices, such as narrow-linewidth external-cavity diode lasers, ultra-narrow filters and demultiplexers.